'Half-Blood Prince' appears poised to beat 'Transformers: Revenge of the Fallen' for the best Wednesday opening. And by Sunday it could break another record.

Remember those records that "Transformers: Revenge of the Fallen" set during its first week at the box office last month? In a few days they could be ancient history.

More than 4,000 sold-out midnight screenings via ticketing websites Fandango and MovieTickets, and massive audience interest, as indicated by pre-release polling, point to a record-breaking opening Wednesday for "Harry Potter and the Half-Blood Prince" in the U.S. and Canada.

"Everything is looking sensational," said Dan Fellman, president of domestic distribution for Warner Bros.

The sixth "Harry Potter" installment cost $250 million to produce and an additional $155 million to market and distribute worldwide, meaning the movie has to reach a high bar at the box office for the studio to recoup its costs and earn a profit.

Precise box-office predictions are difficult for films that, according to pre-release polling, have high appeal across all audience segments. But according to executives who have seen the data, "Half-Blood Prince" has a good shot at beating the all-time domestic record for a Wednesday opening set in June by "Transformers: Revenge of the Fallen." The Michael Bay-directed live-action film took in $62 million in ticket sales on that one day.

And by Sunday, "Potter" could exceed the "Transformer" sequel's $200.1-million gross from Wednesday through Sunday, the record for a movie during that five-day span.

The new "Potter" movie's best chance at breaking records, however, came at the stroke of midnight last night. The record in ticket sales for midnight showings is $18.5 million, set by "The Dark Knight" last summer. As of Tuesday afternoon, Fandango had sold out more than 2,500 midnight showings for "Half-Blood Prince," more than "The Dark Knight," while MovieTickets.com midnight sellouts had exceeded 1,500. Both sites rank the latest "Potter" film as their No. 3 biggest advance ticket seller of all time.

The movie is almost certain to open bigger than "Harry Potter and the Order of the Phoenix," which hit theaters July 11, 2007, and is the only other movie in the series to open on a Wednesday. It earned $44.2 million on its first day and $139.7 million through that Sunday.

That film, like its predecessor, was rated PG-13. However "Half-Blood Prince" is PG, like the first three movies in the series. That means it could play to a somewhat broader audience, although many parents took all but their youngest children to see the last two "Potter" films despite their ratings.

The only thing that could hold back "Half-Blood Prince" is a lack of Imax theaters. "The Dark Knight" played in 94 of the big-screen venues and "Transformers" in 169, but "Half-Blood Prince" is in only three. When Warner Bros. delayed the movie from last November to this week, it landed in a period when most Imax screens are still contracted to show "Revenge of the Fallen."

With their higher ticket prices, Imax screens can be very lucrative. "Transformers" earned $14.4 million from Imax in its first five days.

Warner Bros. will get back most of the Imax screens currently playing "Transformers" the week after next.

"Half-Blood Prince" is also debuting this week in every major international territory, where the film probably will earn even more. "Harry Potter and the Order of the Phoenix" sold $646.2 million worth of tickets overseas, 69% of its worldwide box-office total.